臺北都會區褐碳之光學特性、物種解析與潛在來源之探討

Abstract

近年來褐碳（BrC）因其對空氣品質、氣候變遷及人體健康皆有重大影響而逐漸受到全世界的關注，然而其光學特性、化學組成及來源卻相當複雜，且會隨著時間與空間變化，進而貢獻不同的輻射吸收程度，因此本研究旨在探討BrC之光學特性、發色團組成、潛在來源及其對輻射吸收的貢獻。本研究於臺北都會區採集2021年1月到11月之細懸浮微粒（PM2.5），除了分析其化學組成外，亦量化了分別由水及甲醇萃取出的溶解性有機碳（WSOC和MSOC）之光學特性，此外亦藉由高效能液相層析串聯高解析質譜儀（HPLC-DAD-HRMS）來分析MSOC樣品中的BrC發色團。 根據本研究的結果不僅發現MSOC的光吸收高於WSOC，兩者之光學特性的季節變化亦呈現不同趨勢。WSOC之年質量吸收效率為0.33–1.43 m2 g-1，其中以冬季最大，夏季最小；MSOC則為0.26–1.50 m2 g-1，其中以春季最大和秋季最小；而WSOC和MSOC的年平均吸光埃指數分別為6.05 ± 0.56和5.27 ± 0.59。此兩種萃取物在近紫外光波長範圍內相對於元素碳貢獻的太陽輻射高於在整個太陽光譜範圍內的貢獻，說明了BrC在近紫外光波長處輻射吸收的重要性。本研究亦發現BrC可能源於化石燃料燃燒之交通排放及附近火化場的生質燃燒。此外，已鑑定出的BrC發色團呈現出季節性（冬季、夏季）和事件日與非事件日之間的差異，在冬季及夏季樣品中鑑定出的發色團中包括CHO、CHON及CHOS化合物，其中鑑定出與交通排放、生質燃燒或二次形成有關的苯二甲酸、硝基芳香族化合物、多環芳香烴以及與船隻引擎尾氣排放有關的CHOS化合物；而硝基芳香族化合物和多環芳香烴皆對光吸收有重要之貢獻，其中又以硝基酚類化合物在波長366奈米處的貢獻最為顯著，亦顯示出硝基酚類化合物在BrC光吸收中扮演的重要角色。此外，本研究中鑑定出的多環芳香烴大多以芘為核心結構；而脂肪族化合物則在冬季非事件日與夏季中較常出現。總而言之，本研究的結果揭示臺灣之BrC對當地和區域之空氣品質及氣候有舉足輕重的影響，因此其重要性不容小覷。

Brown carbon (BrC) has been of major concern worldwide due to its significant impact on air quality, climate change and human health. However, the optical properties, chemical constituents and sources of BrC are certainly complex and would change with the variability of spatial and temporal characteristics, leading to different contributions to radiative forcing. Therefore, this study aims to comprehensively investigate the optical properties, chemical constituents and potential sources of BrC. In this study, daily PM2.5 samples were collected from January to November 2021 in the Taipei metropolitan area. The chemical compositions of PM2.5 and optical properties of water-soluble and methanol-soluble organic carbon (WSOC and MSOC) were evaluated and quantified. The BrC chromophores of MSOC were further identified using a high-performance liquid chromatography-diode array detector-electrospray ionization high-resolution mass spectrometer (HPLC-DAD-HRMS) platform. The results showed that the light absorption of MSOC was higher than that of WSOC, while the seasonal variations of the optical properties of WSOC and MSOC were also observed. The annual mass absorption efficiency of WSOC ranged from 0.33 to 1.43 m2 g-1, with the highest value in winter and lowest value in summer, whereas that of MSOC was 0.26–1.50 m2 g-1, with the highest value in spring and lowest value in autumn. The annual average of absorption Ångström exponent of WSOC and MSOC was 6.05 ± 0.56 and 5.27 ± 0.59, respectively. The estimated fractional contributions of radiative forcing of both extracts relative to elemental carbon in the near ultraviolet wavelength (UV) ranges were higher than those in the whole range of the solar spectrum. This suggested the importance of the light absorption of BrC at the near UV wavelengths. The sources of BrC were identified as traffic emissions and biomass burning from adjacent crematoria. In addition, the identified BrC chromophores exhibited the differences between non-polluted winter and summer, as well as non-polluted winter and polluted winter. The compounds identified in both winter and summer included CHO, CHON and CHOS compounds. Phthalic acids (C8H6O4), nitroaromatic compounds (NACs) and polycyclic aromatic hydrocarbons (PAHs) were identified and associated with vehicle emissions and/or biomass burning or secondary formation, while the identified CHOS compounds were pointed out to be generated from ship engine exhaust emissions. NACs and PAHs were observed to significantly contribute to light absorption, especially the light absorption of nitrophenolic compounds (NPs) at the wavelength of 366 nm, highlighting the significant role of these NPs in near UV wavelengths. The light absorption of the compounds identified in winter was most significant at the wavelength of 365 nm, while that in summer was at 300–350 nm. Moreover, the condensed aromatic compounds identified in this study were mostly classified as pyrene core structures. The presence of aliphatic compounds was prevalent during non-polluted winter and summer periods. To sum up, the findings in this study highlight that the significant impact of BrC in Taiwan on local and regional air quality as well as the climate should not be ignored, and need to be further investigated.